The continuous casting of hollow metal shapes is performed in an apparatus provided with a cooled, open-ended mold having a cooled mandrel positioned centrally of the mold cavity. The inner surface of the mold and outer surface of the mandrel cooperate to define a casting space. Molten metal is continuously teemed into the casting space and solidifies adjacent both the inner surface of the mold and the outer surface of the mandrel. This results in a body consisting of a molten mass confined between a pair of spaced, solidified skins.
The body is continuously withdrawn from the casting space thereby generating a continuous, partially solidified strand. The strand is hollow due to the presence of the mandrel in the mold.
In order to accelerate the solidification of the strand, the outer surface of the strand is sprayed with a coolant, typically water, outside of the mold. Once the strand has solidified throughout, it is cut into sections and then subjected to further processing.
One of the problems with the procedure outlined above resides in that the rate of heat extraction from the strand is relatively low inasmuch as heat is removed only via the outer surface of the strand. Since the strand must be solidified throughout before it can be cut, the casting speed must be kept relatively low to insure that complete solidification occurs before the cutting operation. This leads to low production rates. Furthermore, a low rate of heat extraction gives rise to the danger that the inner skin of the strand will remelt thereby permitting the molten metal within the strand to escape. In such an event, the casting operation must be terminated resulting in economic penalties from the points of view of lost material and lost production time.
Another problem with the foregoing procedure stems from the pressure generated by the molten metal within the strand. The pressure may cause the inner skin to bulge or, even worse, to burst, thereby permitting the molten metal to flow out. Bulges on the inner surface of the strand are undesirable from a quality standpoint while the escape of molten metal has the economic consequences indicated earlier. This problem may be avoided for the outer skin of the strand since the outer skin is readily accessible and may be suitably supported if considered necessary to prevent bulging or bursting.
In order to improve the rate of heat extraction from the strand, it has been proposed to spray the inner surface of the strand with water. The water is brought to the interior of the strand via conduits passing through the mandrel. Aside from the benefits which may be realized vis-a-vis casting speed and remelting of the inner skin of the strand, such spraying can lead to an improvement in the internal structure of the strand. However, the problem of bulging and/or bursting of the inner skin of the strand is not overcome by spraying of the inner surface. In the casting of steel, this can have particularly severe consequences since bursting of the inner skin may cause an explosion if the cooling water and molten steel come into contact. In addition, the pipes for the cooling water have sections which extend from the mandrel to the respective water inlet and outlet connections. These pipe sections, of necessity, are located above the bath of molten metal in the mold. Should a break occur so that the cooling water contacts the bath, there may be splattering or, in the case of a steel bath, possibly an explosion. Moreover, since the inner surface of the strand is invisible, it is difficult to determine when changes in the cooling intensity within the strand are required.
A proposal which avoids the danger of explosion and, at least to an extent, the problem of bulging and/or bursting of the inner skin of the strand involves the use of a low-melting point metal as a cooling medium for the inner surface of the strand. Here, a rod having a smaller diameter than the mandrel is aligned with the latter and is located on the longitudinal axis of the strand. The rod, which has an end located within the mold adjacent the mandrel, extends away from the mandrel in the direction of casting. A hollow dummy bar, which is used to start the strand, fits on the rod and is mounted for movement along the same.
The inner surface of the strand and the outer surface of the rod cooperate to define an annular space and the dummy bar has a passage which communicates with this space. The passage is connected to a reservoir via a length of flexible tubing. The reservoir, which has a heat-exchanger and accommodates a low-melting point metal, is also in communication with a central passage formed in the mandrel. The central passage, in turn, opens to the annular space between the rod and the strand. A pump is arranged in the line running from the reservoir to the mandrel.
In operation, a continuously cast strand is started by the dummy bar. Although the dummy bar is connected to the reservoir, it is free to move away from the mold since the connection is formed by flexible tubing. The low-melting point metal contained in the reservoir, which has been heated to a temperature above its melting point, is pumped through the mandrel and into the annular space between the rod and the strand where it absorbs heat from the latter. The liquid metal coolant is subsequently returned to the reservoir via the passage provided for this purpose in the dummy bar. In the reservoir, heat is removed from the liquid metal coolant by means of the heat-exchanger. Thereafter, the liquid metal coolant is recirculated.
While the above apparatus does avoid the danger of explosion and reduce the risk of bulging and/or bursting of the inner skin of the strand, it has the disadvantages of great complexity and high cost. To begin with, the cooling system must be designed with the capability of handling a coolant which is in the form of molten metal. Moreover, not only is it necessary to provide a means for removing heat from the liquid metal coolant but it is further necessary to provide a means for remelting the coolant in the event that the apparatus has been idle for a while. In addition, sliding seals are required between the dummy bar and the rod on which it is mounted in order to prevent leakage of the liquid metal coolant between the dummy bar and the rod. Such seals are difficult to maintain. There are also operational difficulties associated with the foregoing apparatus. Thus, care must be exercised in disconnecting the dummy bar from the strand since, if not done properly, the liquid metal coolant may escape. Also, cooling is no longer possible once the dummy bar has been disconnected from the strand. Additionally, there is no assurance that the gap between the inner surface of the strand and the outer surface of the rod will be uniform. Should the gap, and consequently the cooling effect, be non-uniform, the internal structure of the strand may be adversely affected.
Aside from the foregoing observations regarding the known methods and apparatus, none are intended to control the properties of the inner surface of a continuously cast hollow strand.